1. Field of the Invention
This invention relates to a display device including a flexible printed circuit board and a method of fixing the flexible printed circuit board of the display device.
2. Description of the Related Art
In recent years, in small-sized liquid crystal display devices, there has been employed a chip on glass (COG) method in which a driver IC for driving liquid crystal is mounted on a liquid crystal panel. Here, the liquid crystal panel is connected with a flexible printed circuit board (hereinafter, referred to as FPC board) by pressure bonding through an anisotropic conductive film, and further, a backlight unit using light emitting diodes (LED) is used in most cases.
As technologies related to such a liquid crystal display device including the FPC board, patent documents described below may be exemplified. Hereinafter, devices according to the respective patent documents are briefly described. Members having the same function and portions having similar names in the respective patent documents are denoted by the same reference symbols. However, different reference symbols might be used if there are components to be specially mentioned.
In a liquid crystal display device disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2008-203445, as illustrated in FIGS. 6A and 6B, a bent FPC board 101 is included, and through holes 102 are formed in a bending portion of the FPC board 101. Note that, FIG. 6A is a cross-sectional view of the liquid crystal display device, and FIG. 6B is a developed plane view of the FPC board 101.
When the FPC board 101 is bent, internal stress is generated in a bending region, and brings about a repulsive force or a recovery force due to the bending. This force of the FPC board 101 acts on lifting up a part of a liquid crystal panel 103, which is provided in pressure bonding to the FPC board 101.
Therefore, in order to prevent the lifting of the part of the liquid crystal panel 103, Japanese Unexamined Patent Application Publication (JP-A) No. 2008-203445 proposes a method of decreasing the internal stress by forming the through holes 102 in the FPC board and, as a result, by reducing a sectional area of the FPC board 101, as illustrated in FIG. 6A. This method makes it possible to prevent the lifting phenomenon of the part of the liquid crystal panel 103, which is caused by the repulsive force or the recovery force of the FPC board 101.
Specifically, the liquid crystal display device illustrated in FIG. 6A includes the liquid crystal panel 103, a backlight unit 104, a double-sided pressure sensitive adhesive tape 105, a driver IC 106, a circuit substrate 107, and the FPC board 101.
The liquid crystal panel 103 includes a thin film transistor (TFT) glass substrate 108, a color filter glass substrate 109, and liquid crystal (not shown) provided between the substrates. Polarizing plates 110 and 111 are disposed on an upper side and a lower side of the liquid crystal panel 103, respectively.
The backlight unit 104 includes a frame 112, an LED 113, a light guide plate 114, a reflection film 115, and optical films 116 and 117.
The double-sided pressure sensitive adhesive tape 105 is a rectangular shape, and is provided between the liquid crystal panel 103 and the backlight unit 104 to fix the liquid crystal panel 103 and the backlight unit 104. The backlight unit 104 is adhered and fixed onto the circuit substrate 107 by a double-sided pressure sensitive adhesive tape 118. One end of the FPC board 101 is pressure bonded to the liquid crystal panel 103, and another end of the FPC board 101 is pressure bonded to the circuit substrate 107.
The FPC board 101 is bent in the example above, and hence internal stress is generated. Therefore, an FPC pressure-bonding portion of the liquid crystal panel 103 receives a force applied in the upper direction in the plane of the sheet of FIG. 6A. When this force is strong, it often happens that the liquid crystal panel 103 is separated from the double-sided pressure sensitive adhesive tape 105 to be lifted up.
Therefore, in Japanese Unexamined Patent Application Publication (JP-A) No. 2008-203445, the through holes 102 are formed in the FPC board 101, to thereby reduce the sectional area of the region in which the internal stress is generated. In this manner, the internal stress is reduced. As a result, the lifting of the liquid crystal panel 103 may be prevented.
In addition, Japanese Unexamined Patent Application Publication (JP-A) No. 2007-323017 discloses a liquid crystal display device, as illustrated in FIG. 7. In the illustrated liquid crystal device, a prominence 131 is formed on a holder 130, and the prominence 131 is inserted through a through hole 133 formed in an FPC board 132. Note that, FIG. 7 is a cross-sectional view of the liquid crystal display device.
In FIG. 7, the prominence 131 formed on a side surface of the holder 130 is helpful to suppress a warp of the FPC board 132 in the upper direction (upper side in the plane of the sheet of FIG. 7). Therefore, the FPC board 132 may be prevented from being separated from the liquid crystal panel or being detached from the holder 130.
Japanese Unexamined Patent Application Publication (JP-A) No. 07-38268 discloses a housing assemble structure of a flexible unit, which is similar in structure to that of Japanese Unexamined Patent Application Publication (JP-A) No. 2007-323017. In Japanese Unexamined Patent Application Publication (JP-A) No. 07-38268, a hole is formed in an FPC board, and the hole is engaged with a shaft formed in a housing, to thereby absorb an external force applied to the FPC board. In this manner, solder connection portions of the FPC board and a substrate are prevented from being separated from each other.
In addition, an FPC board 143 in Japanese Unexamined Patent Application Publication (JP-A) No. 10-135596 has, as illustrated in FIGS. 8A and 8B, a through hole 141 formed therein near an FPC connection terminal 140. A land (reinforcement land) 142 is formed in the through hole 141. Note that, FIG. 8A is a partial plane view of the FPC board 143 in a region near the FPC connection terminal 140, and FIG. 8B is a cross-sectional view of this part taken along the line passing through the through hole 141.
As is understood from FIGS. 8A and 8B, a connection terminal 145 of a circuit substrate 144 and the land 142 are soldered to each other with solder 146. In this manner, a soldering strength is reinforced to prevent separation of the FPC connection terminal 140.
Japanese Unexamined Patent Application Publication (JP-A) No. 2008-84452 discloses an optical pick-up using a technology similar to that of Japanese Unexamined Patent Application Publication (JP-A) No. 10-135596. That is, a PD flexible printed board and an optical pick-up (OPU) circuit substrate are fixed to each other at three points to prevent occurrence of separation.
Specifically, a central reinforcing land and both-end reinforcing lands are formed on the OPU circuit substrate, and a central circular hole and both-end connecting parts are correspondingly formed on the PD flexible printed board. The central reinforcing land and the central circular hole, and each both-end reinforcing land and the corresponding both-end connecting part are fixed to each other by soldering, to thereby improve connection strength.
In addition, in a display device disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2008-76930, as illustrated in FIG. 9, an FPC board 151 is connected to a (liquid crystal display) substrate 150, and a through hole 152 formed in the substrate 150 and a through hole 153 formed in the FPC board 151 are aligned with each other. In this state, the substrate 150 and the FPC board 151 are fixed to each other by a joint member 154 passing through both of the through holes 152 and 153. With this structure, separation due to bending of the FPC board 151 may be prevented. Note that, the joint member 154 is formed by a thermoplastic material, a high polymer material, a metal material, a bolt, and a nut.
In addition, Japanese Unexamined Patent Application Publication (JP-A) No. 10-9224 discloses a method of fixing and reinforcing an FPC board 160 by using a pair of fixation clips as illustrated in FIG. 10. Through holes 162 are formed in a substrate 161, and through bores (holes) 163 are formed in the FPC board 160 at corresponding positions.
Under a state in which both of the through holes 162 and 163 are aligned, the substrate 161 and the FPC board 160 are soldered to each other. Then, a pin 165 of one clip 164 is inserted through one pair of the through holes 162 and 163 from an upper side of the FPC board 160, and a pin 167 of another clip 166 is inserted through another pair of the through holes 162 and 163 from a lower side of the substrate 161. In this state, the pins 165 and 167 are fitted into sockets 168 and 169 of the opposing clips 166 and 164, respectively. With this structure, separation between the FPC board 160 and the substrate 161 due to external stress may be prevented.
Japanese Unexamined Patent Application Publication (JP-A) No. 2002-204068 discloses a technology related to a method of jointing a printed wiring board and an FPC board, which is similar to that of Japanese Unexamined Patent Application Publication (JP-A) No. 10-9224. In Japanese Unexamined Patent Application Publication (JP-A) No. 2002-204068, as illustrated in FIG. 11, pin holes 172 and 173 are formed at joint portions of an FPC board 170 and a printed wiring board 171, respectively, and the printed wiring board 171 and the FPC board 170 are sandwiched between a pin plate 174 and a holed plate 175 in which a pin hole 177 is formed.
In this manner, a pin 176 of the pin plate 174 is caused to pass through the respective pin holes 172, 173, and 177 of the FPC board 170, the printed wiring board 171, and the holed plate 175, and after that, a tip end of the pin 176 is clamped. With this, separation in a case where stress is applied to the FPC board 170 is prevented.
In addition, in a plasma display module disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2005-122174, as illustrated in FIG. 12, in order to bring a grounded line 180 and a chassis base 181 in intimate contact to each other, a wiring 182, specifically, a through bore (hole) 183 of an FPC board is formed. Further, a coupling hole 184 is formed in the chassis base 181, and a coupler as pressure means 185, for example, a screw is screwed therein.
However, when the FPC board is fixed to a support portion of the liquid crystal display device in a bent state, due to a force generated in the FPC board by recovery from deformation, that is, spring-back, troublesome phenomena described below occur in some cases.
Specifically, in a liquid crystal display device including a backlight unit, a liquid crystal panel, an FPC board for backlight, which is connected to the backlight unit, and an FPC board for liquid crystal panel, which is connected to the liquid crystal panel, in a case where the FPC board for liquid crystal panel is bent to be fixed to a back surface of the backlight unit being the support portion by a double-sided pressure sensitive adhesive tape, there occurs, in some cases, a troublesome phenomenon that the FPC board for liquid crystal panel is separated from the double-sided pressure sensitive adhesive tape adhered onto the back surface of the backlight unit.
Moreover, there occurs, in some cases, a troublesome phenomenon that the liquid crystal panel is lifted up from the backlight unit.
Further, when the FPC board for backlight is used in a bent state, there occurs, in some cases, a troublesome phenomenon that the LED fixed onto the FPC board for backlight is displaced, which causes uneven brightness.
Further, in Japanese Unexamined Patent Application Publication (JP-A) No. 2008-203445, the through holes 102 are formed in the FPC board 101 to reduce the sectional area thereof, and accordingly reduce the internal stress. However, as illustrated in FIG. 6B, a size or position of each of the through holes 102 capable of being formed in the FPC board 101 for reduction of the sectional area is limited because a conductive pattern 119 exists. Therefore, an effect of reducing the internal stress of the FPC board 101 is limited.
In Japanese Unexamined Patent Application Publication (JP-A) No. 2007-323017, when the holder 130 is stored in the frame, the FPC board 132 is also stored in the frame, and hence it is necessary to shape the frame structure so that an FPC terminal can be taken out to the outside.
Further, as illustrated in FIG. 7, the LED 113 is mounted on an upper side of the FPC board 132, and the reflection film 115 is not provided directly under the LED 113. In order to improve the brightness, it is preferred that the LED 113 be mounted on a lower side of an FPC board for LED, and the reflection film 115 be provided directly under the LED 113.
In this case, it is difficult to integrally form the FPC board for LED, that is, the FPC board for backlight, and the FPC board for liquid crystal panel as one FPC board. Further, when an FPC terminal for backlight and an FPC terminal for liquid crystal panel are taken out of the frame, terminal connectors are necessary for the FPC board for backlight and the FPC board for liquid crystal panel.
In Japanese Unexamined Patent Application Publication (JP-A) No. 10-135596, in a case where the circuit substrate 144 illustrated in FIG. 8B is replaced by the backlight unit, it is necessary to add a step of forming a land for solder on the backlight unit and a step of soldering the land with the through hole of the FPC board.
In Japanese Unexamined Patent Application Publication (JP-A) No. 2008-76930, in a case where the substrate 150 illustrated in FIG. 9 is replaced by the backlight unit, a backlight unit through hole and an FPC through hole are formed, and an I-shaped joint member passing through both of the through holes is used for fixation.
In this case, it is necessary to cause the I-shaped joint member to extend beyond the lower side of the backlight unit through hole. This extending portion of the I-shaped joint member is responsible for an increase in thickness of the liquid crystal display device.
In Japanese Unexamined Patent Application Publication (JP-A) No. 10-9224, in a case where the substrate 161 illustrated in FIG. 10 is replaced by the backlight unit, backlight unit through holes and FPC through holes are formed, and the pins of the pair of clips are inserted through the through holes in the substrate and the FPC through holes. Further, the pins are fitted into the sockets of the opposing clips, respectively. In this case, the clip on the lower side of the backlight unit is responsible for an increase in thickness of the liquid crystal display device.
Similarly, in Japanese Unexamined Patent Application Publication (JP-A) No. 2002-204068, in a case where the printed wiring board 171 illustrated in FIG. 11 is replaced by the backlight unit, pin holes are formed at joint portions of the backlight unit and the FPC board, and the pin of the pin plate is caused to pass through the respective pin holes of the backlight unit, the FPC board, and the holed plate, and after that, the tip end of the pin is clamped. In this case, the pin plate or the holed plate is inevitably increased in thickness of the liquid crystal display device.
In Japanese Unexamined Patent Application Publication (JP-A) No. 2005-122174, in a case where the chassis base 181 illustrated in FIG. 12 is replaced by the backlight unit, a hole is formed in the FPC board to screw the FPC board to the backlight unit with a screw. In this manner, rigid fixation may be achieved. However, in this case, a device such as an electric screwdriver is necessary, and also it is necessary to add a step of fixing the screw by rotating the screw.